Methods:Respiratory-correlated (4D) PET/CT was used for this proof-of-concept study. Amplitude-gated 4D-CT scans of 2 lung cancer patients acquired as part of an IRB-approved protocol were studied. List-mode FDG-PET data was acquired in the same sessions and gated into 4D images (i.e., activity distributions) matched in phase to the 4D-CT. End-inhale phase 4D-CT images were deformably registered to the end-exhale phase images using a free-form intensity based algorithm provided by a commercial software package. The corresponding 4D-PET activity distributions at the same inhale breathing phase were deformed using the deformation vector fields from the 4D-CT registration. CT registration accuracy was then assessed by visually inspecting the deformed inhale-phase PET activity distributions against the exhale-phase activity distributions. Errors in registration were identified when the deformed inhale-phase activity distribution failed to match the exhale phase distribution.

Results:To test the utility of this methodology deformed PET activity distributions were used to identify CT-CT registration failure in the region of the mediastinum. The registration failure could not be seen by inspection of the deformed CT itself, due to lack of contrast inside the mediastinum, but was clearly visible from the deformation of the activity distribution of a tracer-avid lymph node. Control cases were used for validation, consisting of correct and incorrect registrations of lung tumors near the chest wall, where the performance of the registration was visible from CT deformation alone.

Conclusion:PET-measured tracer activity in combined PET/CT scans can be used as a ground truth to qualitatively assess the accuracy of deformable image registration of CT images and deformable dose accumulation. Further work is needed to understand the limits of quantitative assessment.

Funding Support, Disclosures, and Conflict of Interest: This work is supported in part by NIH R01CA096679 and R01CA116712.